1. Field of the Invention
The present invention relates generally to crankcases that convert an input rotary motion into an output linear motion, such as is required for piston pumps, and more specifically to mechanisms that vary the stroke length of such piston pumps.
2. Description of Related Art
A variety of machines require a linear, oscillating input motion for operation. For example, positive displacement piston pumps require an linear, oscillating motion to drive their pistons in and out of the piston chamber to displace a control volume within the chamber. Such piston pumps are used in farm machinery such as liquid-fertilizer distribution systems to disperse a controlled volume of liquid fertilizer.
While such machines require a linear, oscillating power source for operation, mechanical power sources are typically rotational. For example, motors and engines are typically rotational power sources. In farm machinery, for example, various implements must be powered by a rotational power take-off.
The rotational power source in farm machinery may also be a passive ground drive system that includes a ground-engaging driving wheel that rotates as the machinery is pulled over the ground. This rotation is transferred to a driveshaft through gears, belt drives, or other suitable means.
A mechanism is therefore required for converting the rotary motion input from a source, such as a chain or belt driven sprocket, to a linear motion output for use in such machines as positive displacement piston pumps. One conventional conversion mechanism utilizes a connector rod with a first end pivotally mounted to a piston of a piston pump or other linear-motion requiring machine. As a result, the first end of the connector is restricted to motion along a line that is parallel to the cylinder""s axis. A second end of the connector rod is connected to a driveshaft at a pivot point that is offset from the rotational axis of the driveshaft. In practice, this is often accomplished by mounting an offset hole of an eccentric to the driveshaft. The second end of the connector rod is then mounted onto the outer cylindrical surface of the eccentric. When this eccentric system is used, the first end of the connector rod moves in a circular path around the driveshaft axis and forces the second end of the connector rod to drive the piston pump.
It is desirable to be able to selectively vary the flow rate through such machinery as piston pumps. In the case of a piston pump, the flow rate can be altered by either changing the stroke frequency or stroke length. In many situations, the speed of the input rotational power supply cannot be readily adjusted in order to adjust the resulting piston stroke frequency. For example, in the case of a ground-drive-powered piston pump, the speed of the driveshaft is controlled solely by the machinery""s speed over land. In order for an operator to have selective control over the output flow rate of the piston pump, the operator must therefore be able to adjust the stroke length of the piston.
One conventional method of varying the output stroke length is to use nested locking eccentrics instead of a single eccentric as discussed above. In this case, a first eccentric having an offset hole is rotationally fixed to the driveshaft. A second eccentric has an offset hole that fits over the outer surface of the first eccentric. The eccentrics are variably rotationally fixed to each other such that a user can select their relative positions in order to alter the effective offset between the second end of the connector rod and the driveshaft axis. As a result, the stroke length of the first end of the connector is variable. Unfortunately, however, when using this conventional system, the machine must be stopped in order to allow an operator to unlock, alter, and relock the eccentrics"" relative positions.
The variable rate pump according to the present invention is unique in its ability to vary its pumped output at constant input rpm by varying its stroke length or linear motion output from the crankcase on-the-fly.